Compositions comprising a mixture of a BAM phosphor and at least one other hexaaluminate

ABSTRACT

Disclosed herein are phosphor compositions comprising a BAM phosphor BaMgAl 10 O 17 :Eu 2+  and at least one other hexaaluminate that may also be europium activated. The BAM may be mixed with any number of different kinds of heaxaaluminates having a similar crystal structure. The aluminum ratio may also be adjusted to alter the defect structure or to produce a second phase. Addition of another hexaaluminate to BAM enhances emission intensity and resistance to degradation, which is beneficial to applications such as plasma display panels.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 60/830,269, filed Jul. 11, 2006, and titled “An improved BAMphosphor.” U.S. Provisional Application Ser. No. 60/830,269 isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention are directed to phosphorcompositions that comprise a BAM phosphor BaMgAl₁₀O₁₇:Eu²⁺ and at leastone other hexaaluminate that may also be europium activated.

2. Description of the Related Art

Owing to its high light output and excellent display of color (asrepresented, for example, on a CIE color diagram), the known BAMphosphor BaMgAl₁₀O₁₇ doped with divalent europium (Eu²⁺) has been widelyused as the blue phosphor component in applications such as fluorescentlamps, light emitting diode (LED) and plasma display panels (PDP). Inplasma display panel applications, BAM:Eu is conventionally adopted asthe blue-emitting component under vacuum ultraviolet (VUV) excitation;however, considerable degradation in light output and color shift(toward the green) are known to be problematic. This is thought to bedue to an annealing procedure during the manufacturing process, as wellas to plasma radiation and/or sputtering damage that occurs duringday-to-day use.

BAM has a β-alumina structure, and belongs to the family ofhexaaluminates. Hexaaluminates have a column-like structure and consistsof blocks of cubic closed packed (CCP) oxygen layers with cations intetrahedral and octahedral interstices. Because the blocks are quitesimilar to the MgAl₂O₄ spinel structure, hexyluminates are oftenreferred to as “spinel blocks.” The blocks are separated by mirrorplanes, which contain the large cations.

The magnetoplumbite structure is similar to the β-alumina structure asboth have identical spinel blocks. The difference between them lies inthe mirror planes, which are loosely-packed in the magnetoplumbite case,and tightly-packed in the β-alumina structure. LaMgAl₁₁O₁₉ is a typicalmagnetoplumbite. Structurally, its formula can be rewritten as[LaAlO₃][(Al₃Mg)A₇O₁₆], where [LaAlO₃] are the ions on the mirror plane,and the [(Al₃Mg)Al₇O₁₆] portion of the structure exists as theabove-mentioned spinel blocks. In the formula (Al₃Mg) are groups of ionshaving a 4-fold coordination, and the aluminum as part of the Al₇ oxidehas a six-fold coordination. In the same manner, β-alumina BaMgAl₁₀O₁₇can be rewritten as [BaO][(Al₃Mg)A₇O₁₆], where just two ions Ba and Oare presented on the mirror plane.

Diagrams of the atomic arrangements (and hence crystal structure) ofthree hexaaluminates are shown in FIGS. 1A-1C. FIG. 1A is β-alumina asrepresented by the formula NaAl₁₁O₁₇, which may be re-written as[NaO][Al₄Al₇O₁₆]; FIG. 1B is the BAM compound BaMgAl₁₀O₁₇, which may bewritten as [BaO][(Al₃Mg)Al₇O₁₆]; and FIG. 1C is the magnetoplumbiteLaMgAl₁₁O₁₉, which may be written as [LaAlO₃][(Al₃Mg)Al₇O₁₆].

What is needed in the art are phosphor compositions that enhance theemission intensity and degradation resistance of the conventional BAMphosphor (BaMgAl₁₀O₁₇:Eu²⁺), utilizing properties afforded by mixing theconventional BAM phosphor with other hexaaluminates.

SUMMARY OF THE INVENTION

Embodiments of the present invention are directed to a phosphorcomposition comprising a europium activated BAM phosphor and ahexaaluminate other than the BAM phosphor, the composition representedby the formula (x) hexaaluminate+(1−x) BaMgAl₁₀O₁₇:Eu²⁺, where thehexaaluminte is selected from the group consisting of a β-alumina, aβ′-alumina, and a magnetoplumbite, and wherein x ranges from about 0.001to about 0.999.

In another embodiment, the phosphor composition comprises a europiumactivated BAM phosphor and a hexaaluminate other than the BAM phosphor,the composition represented by the formula (x) LnMAl₁₁O₁₉+(1−x)BaMgAl₁₀O₁₇:Eu²⁺, where Ln is a trivalent lanthanide, M is a divalentcation, and x ranges from about 0.001 to about 0.5. Alternatively, thephosphor composition comprises a europium activated BAM phosphor and ahexaaluminate other than the BAM phosphor, the composition representedby the formula (x) Ln_(u)Al_(v)O_(w)+(1−x) BaMgAl₁₀O₁₇:Eu²⁺, where Ln isa trivalent lanthanide, x ranges from about 0.001 to about 0.5, u rangesfrom about 0.67 to about 1, v ranges from about 11 to about 12, and wranges from about 18 to about 19.

In another embodiment, the phosphor composition comprises a europiumactivated BAM phosphor and a hexaaluminate other than the BAM phosphor,the composition represented by the formula (x)M′_(1.5)Al_(10.5)O_(16.5)+(1−x) BaMgAl₁₀O₁₇:Eu²⁺. Alternatively, thephosphor composition comprises a europium activated BAM phosphor and ahexaaluminate other than the BAM phosphor, the composition representedby the formula (x) M′_(1.5)Al_(10.5)O_(16.5)+(1−x) BaMgAl₁₀O₁₇:Eu²⁺,where M′ is a monovalent cation, and x ranges from about 0.001 to about0.5.

In another embodiment, the phosphor composition comprising a europiumactivated BAM phosphor and a hexaaluminate other than the BAM phosphor,the composition represented by the formula (x)M_(0.75)Al₁₁O_(17.25)+(1−x) BaMgAl₁₀O₁₇:Eu²⁺, where M is a divalentcation, and x ranges from about 0.001 to about 0.5. Alternatively, thephosphor composition comprising a europium activated BAM phosphor and ahexaaluminate other than the BAM phosphor, the composition representedby the formula x (yMO.6Al₂O₃)+(1−x) BaMgAl₁₀O₁₇:Eu²⁺, where M is adivalent cation, and x ranges from about 0.001 to about 0.5, and yranges from about 1.28 to about 1.32.

The present phosphor compositions may be synthesized by a methodselected from the group consisting of liquid processing methods,co-precipitation methods, and sol-gel methods. Exemplary steps include:(a) dissolving the precursor metal salts in an aqueous based solution;(b) co-precipitating an intermediate product; (c) removing at least aportion of the water the intermediate product of step (b); (d) calciningthe product of step (c); and (e) sintering the product of step (d).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C are diagrams showing the atomic arrangements (and hencecrystal structure) of three hexaaluminates: FIG. 1A is β-alumina asrepresented by the formula NaAl₁₁O₁₇, which may be re-written as[NaO][Al₄Al₇O₁₆]; FIG. 1B is the BAM compound BaMgAl₁₀O₁₇, which may bewritten as [BaO][(Al₃Mg)Al₇O₁₆]; and FIG. 1C is the magnetoplumbiteLaMgAl₁₁O₁₉, which may be written as [LaAlO₃][(Al₃Mg)Al₇O₁₆];

FIG. 2 is a graph of emission intensity vs. fraction of alumina (denotedby “x”), comparing heated and unheated samples;

FIG. 3 is an x-ray diffraction (XRD) pattern of the compound(Na_(0.2)Ba_(0.75)Eu_(0.05)Mg_(0.8)O_(1.7))x(Al_(10.2)O_(15.3)), where xis greater than or equal to 100 percent (x≧100%); and

FIG. 4 is an x-ray diffraction (XRD) pattern of the compound(Na_(0.2)Ba_(0.75)Eu_(0.05)Mg_(0.8)O_(1.7))x(Al_(10.2)O_(15.3)), where xis less than or equal to 100 percent (x≦100%).

DETAILED DESCRIPTION OF THE INVENTION

Disclosed herein are phosphor compositions comprising a BAM phosphorBaMgAl₁₀O₁₇ activated with divalent europium (Eu²⁺), and at least oneother hexaaluminate having a similar crystal structure. The similarityamong hexaaluminte structures has led the present inventors tosynthesize new compositions by mixing various hexaaluminates.

Exemplary Formulations

According to embodiments of the present invention, a BAM:Eu²⁺ is mixedwith at least one other type of hexaaluminate, as represented by thegeneral formula:

(x)hexaaluminate+(1−x)BaMgAl₀O₁₇:Eu²⁺

where the (x) hexaaluminte includes but is not limited to a β-alumina, aβ′-alumina, and a magnetoplumbite, and is not BaMgAl₁₀O₁₇. The phosphorcomposition mixture may be in the form of a solid solution of thehexaaluminate and the BAM:Eu²⁺, or it may contain distinct phases of thehexaaluminate and the BAM:Eu²⁺. The value of x in this general formularanges from about 0.001 to about 0.999.

More specifically, and according to one embodiment of the presentinvention, the phosphor composition is generated by mixing the BAMphosphor with a magnetoplumbite compound, the phosphor compositionrepresented by the formula:

(x)LnMAl₁₁O₁₉+(1−x)BaMgAl₁₀O₁₇:Eu²⁺,

where Ln is a trivalent lanthanide, M is a divalent cation, and x rangesfrom about 0.001 to about 0.5. In some embodiments, M may be an alkalineearth metal from group IIA of the periodic table, the alkaline earthmetal selected from the group consisting of Mg, Ca, Sr, and Ba.

According to another embodiment of the present invention, the BAMphosphor may be mixed with a lanthanide-containing hexaaluminatecompounds according to the formula:

(x)Ln_(u)Al_(v)O_(w)+(1−x)BaMgAl₁₀O₁₇:Eu²⁺,

where Ln is a trivalent lanthanide, x ranges from about 0.001 to about0.5, u ranges from about 0.67 to about 1, v ranges from about 11 toabout 12, and w ranges from about 18 to about 19.

While not wishing to be bound by any particular theory, it may be notedthat the lanthanide hexaaluminate has a magnetoplumbite framework of thetype AB₁₂O₁₉, with vacancies in the structure. Ideally, the structurewould have the formula Ln_(0.67)Al₁₂O₁₉, in accordance with a “true”magnetoplumbite structure. However, such structures apparently do notexist as the AB₁₂O₁₉ framework cannot accommodate a sufficient number ofvacancies at the A sites. Therefore, it is believed the actualcomposition of a lanthanide hexaaluminate lies somewhere between thestoichiometric LnAl₁₁O₁₈, and the ideal stoichiometry ofLn_(0.67)Al₁₂O₁₉. An example of such a lanthanide hexaaluminate isLa_(0.85)Al_(11.6)O_(18.7).

In another embodiment of the present invention, the BAM phosphor may bemixed with a hexaaluminate comprising one or more β-alumina compoundssuch that the phosphor composition has the formula:

(x)M′Al₁₁O₁₇+(1−x)BaMgAl₁₀O₁₇:Eu²⁺,

where M′ is a monovalent cation from group IA of the periodic table (analkali metal), and x ranges from about 0.001 to about 0.5. The M′ cationis selected from the group consisting of Li, Na, K, Rb, and Cs.

In another embodiment of the present invention, the BAM phosphor may bemixed with a hexaaluminate comprising one or more of the so-calledβ′-alumina compounds such that the phosphor composition has the formula:

(x)M′_(1.5)Al_(10.5)O_(16.5)+(1−x)BaMgAl₁₀O₁₇:Eu²⁺,

where M′ is a monovalent cation from group IA of the periodic table (analkali metal), and x ranges from about 0.001 to about 0.5. The M′ cationis selected from the group consisting of Li, Na, K, Rb, and Cs. Theassumed structure of β′-alumina has been reported before in theliterature; however, whether it is a new phase other thannon-stoichiometric β-alumina remains unclear.

In another embodiment of the present invention, the BAM phosphor may bemixed with one or more alkaline-earth-poor (or alkaline earth deficient,at least relative to previous embodiments) hexaaluminate compounds, suchthat the phosphor composition has the formula:

(x)M′_(0.75)Al₁₁O_(17.25)+(1−x)BaMgAl₁₀O₁₇:Eu²⁺,

where M is a divalent cation, and x ranges from about 0.001 to about0.5. The alkaline-earth-poor hexaaluminates have a β-alumina structurewith about 75 percent of the group IA alkali metal ions being replacedby group IIA alkaline-earth ions and about 25 percent by oxygen ions.

In another embodiment of the present invention, the BAM phosphor may bemixed with one or more alkaline-earth-rich hexaaluminate compounds, suchthat the phosphor composition has the formula:

x(yMO.6Al₂O₃)+(1−x)BaMgAl₁₀O₁₇:Eu²⁺,

where M is a divalent cation, and x ranges from about 0.001 to about0.5, and y ranges from about 1.28 to about 1.32. Thesealkaline-earth-rich hexaaluminates are assumed to have a β′-aluminastructure with about 75 percent of the group IA alkali metal ions beingreplaced by group IIA alkaline-earth ions and about 25% by oxygen ions.This embodiment provides a composition with the ideal structural formulaM_(1.125)Al_(10.5)O_(16.875).

For certain mixtures of BAM:Eu²⁺ and some other hexaaluminte, accordingto the embodiments outlined above, the ratio of the aluminum to theother cations may be varied to enhance luminescence output and oxidationstability. Furthermore, it is believed that by mixing BAM:Eu²⁺ withother hexaaluminates according to the present embodiments, oxidativestability and light emission output are enhanced due to changes in thecrystal field properties of the overall phosphor composition.

Processing Considerations

The present phosphor compositions may be synthesized by mixing the BAMphosphor BaMgAl₁₀O₁₇ with one or more hexaaluminates other than the BAMhaving a similar crystal structure.

In one embodiment of the present invention, the phosphor composition maybe synthesized by mixing the BAM phosphor BaMgAl₁₀O₁₇ with the β-alumina(NaAl₁₁O₁₇). The fraction of the β-alumina contained within thecomposition may be adjusted to vary the light emission behavior anddegradation resistance of the overall composition. In this embodiment acomposition is mixed having a content of about 20 percent of theβ-alumina NaAl₁₁O₁₇ and about 80 percent of the BAM. The formula of thecomposition may be represented by the formula:

(Na_(0.2)Ba_(0.75)Eu_(0.05)Mg_(0.8)O₁₇)+x(Al_(10.2)O_(15.3)),

where x ranges from about 0.7 to about 1.30.

In one example of the synthesis of the present phosphor composition, thestarting materials comprised the appropriate metal nitrates in thedesired mole ratios. In one embodiment, a flux may be added duringprocessing, for example, a 5 mole percent addition of a flux such asaluminum fluoride.

Such BAM/hexaaluminate compositions have been synthesized by the presentinventors using liquid mixing, co-precipitation, and/or sol-geltechniques. In accordance with these processes, metals that includedsodium, barium, magnesium, aluminum, and europium, along with salts ofhalogens such as aluminum fluoride, were first dissolved in hot water.An aqueous solution of ammonia water was added to facilitateco-precipitation of the mixed nitrates. The solution was then heated toremove water, and the partially dried mixture was calcined at about 800°C. for about two hours. Finally, the calcined powders were sintered atabout 1500° C. for about 6 hours in an atmosphere comprising nitrogengas mixed with about 1 to 5 percent by volume hydrogen. After sintering,the powders were milled and sieved with a 25 μm sieve.

Physical Properties and Optical Performance

To perform a thermal degradation test, the powders were heated at about510° C. for about one hour in air. The emission intensity of un-heatedand heated samples were then measured using a 147 nm plasma lamp as theexcitation source. Sample compositions and measurement data are shown inTable 1:

TABLE 1 Compositions and photoemission intensities of samples(Na_(0.2)Ba_(0.75)Eu_(0.05)Mg_(0.8)O_(1.7)) + (Al_(10.2)O_(15.3))Emission Emission Emission change Sample # x Compositions (unheated)(heated) ratio Control (Ba_(0.95)Eu_(0.05))MgAl₁₀O₁₇ (BAM) 1097 1060−4.6% 1 70% Na_(0.2)Ba_(0.75)Eu_(0.05)Mg_(0.8)Al_(7.14)  952  975 +2.4%2 80% Na_(0.2)Ba_(0.75)Eu_(0.05)Mg_(0.8)Al_(8.16) 1202 1191 −0.91%  390% Na_(0.2)Ba_(0.75)Eu_(0.05)Mg_(0.8)Al_(9.18) 1106 1102 −0.36%  4 95%Na_(0.2)Ba_(0.75)Eu_(0.05)Mg_(0.8)Al_(9.69) 1133 1102 −2.7% 5 100%Na_(0.2)Ba_(0.75)Eu_(0.05)Mg_(0.8)Al_(10.2) 1116 1058 −5.2% 6 105%Na_(0.2)Ba_(0.75)Eu_(0.05)Mg_(0.8)Al_(10.71) 1091 1090 −0.1% 7 110%Na_(0.2)Ba_(0.75)Eu_(0.05)Mg_(0.8)Al_(11.22) 1067 1069 +0.2% 8 120%Na_(0.2)Ba_(0.75)Eu_(0.05)Mg_(0.8)Al_(12.24) 1059 1044 −1.4% 9 130%Na_(0.2)Ba_(0.75)Eu_(0.05)Mg_(0.8)Al_(13.26) 1064 1036 −2.6%

These experiments show that of the data presented, the compositionscomprising mixtures of the conventional BAM phosphor(Ba_(0.95)Eu_(0.05))MgAl₁₀O₁₇ with hexaaluminates other than that BAM inall cases demonstrate either an increase in intensity, or at least lessof an intensity decrease from heating than the control.

FIG. 2 is a graph of emission intensity versus x, the fraction ofalumina in the composition. In the graph, the square symbols representunheated samples, and the circles heated samples. Data in the figureshows that the emission intensity increases dramatically as the fractionof the alumina in the composition is increased from 0.7 to 0.8, andwhereas it decreases somewhat from that highest value, the emissionintensity is still greater for x fractions of 0.9 to 1.3 than theemission intensity is when x is 0.7.

FIG. 3 is an x-ray diffraction (XRD) pattern of the compound(Na_(0.2)Ba_(0.75)Eu_(0.05)Mg_(0.8)O_(1.7))x(Al_(10.2)O_(15.3)), where xis greater than or equal to 100 percent (x≧100%). The diffractionpattern shows a comparison of samples with increasing ratios ofaluminum. The data shows that as a second phase of α-alumina becameevident as the value of x was increased above 100 percent. The largerthe value of x, the more prominent the diffraction peak(s) of theα-alumina became.

FIG. 4 is an x-ray diffraction (XRD) pattern of the compound(Na_(0.2)Ba_(0.75)Eu_(0.05)Mg_(0.8)O_(1.7))x(Al_(10.2)O_(15.3)), where xis less than or equal to 100 percent (x≦100%). In FIG. 4, thediffraction pattern shows a comparison of samples with smaller ratios ofaluminum than found in stoichiometrical composition. It was found that asecond phase, BaAlO₂, was formed when x was decreased to about 70percent of the stoichiometrical ratio. When x was equal to or less thanabout 80 percent, no second phase was formed. Furthermore, the β-aluminacould not be distinguished from the BAM in the diffraction pattern.Therefore, it could not be determined in this case whether or notβ-alumina existed in the form of a second phase.

1. A phosphor composition comprising a europium activated BAM phosphorand a hexaaluminate other than the BAM phosphor, the compositionrepresented by the formula:(x)hexaaluminate+(1−x)BaMgAl₁₀O₁₇:Eu²⁺ where the hexaaluminte isselected from the group consisting of a β-alumina, a β′-alumina, and amagnetoplumbite, and wherein x ranges from about 0.001 to about 0.999.2. A phosphor composition comprising a europium activated BAM phosphorand a hexaaluminate other than the BAM phosphor, the compositionrepresented by the formula:(x)LnMAl₁₁O₁₉+(1−x)BaMgAl₁₀O₁₇:Eu²⁺, where Ln is a trivalent lanthanide,M is a divalent cation, and x ranges from about 0.001 to about 0.5. 3.The phosphor composition of claim 2, wherein M is an alkaline earthmetal selected from the group consisting of Mg, Ca, Sr, and Ba.
 4. Aphosphor composition comprising a europium activated BAM phosphor and ahexaaluminate other than the BAM phosphor, the composition representedby the formula:(x)Ln_(u)Al_(v)O_(w)+(1−x)BaMgAl₁₀O₁₇:Eu²⁺, where Ln is a trivalentlanthanide, x ranges from about 0.001 to about 0.5, u ranges from about0.67 to about 1, v ranges from about 11 to about 12, and w ranges fromabout 18 to about
 19. 5. A phosphor composition comprising a europiumactivated BAM phosphor and a hexaaluminate other than the BAM phosphor,the composition represented by the formula:(x)M′Al₁₁O₁₇+(1−x)BaMgAl₁₀O₁₇:Eu²⁺, where M′ is a monovalent cation, andx ranges from about 0.001 to about 0.5.
 6. The phosphor composition ofclaim 5, wherein the M′ cation is selected from the group consisting ofLi, Na, K, Rb, and Cs.
 7. A phosphor composition comprising a europiumactivated BAM phosphor and a hexaaluminate other than the BAM phosphor,the composition represented by the formula:(x)M′_(1.5)Al_(10.5)O_(16.5)+(1−x)BaMgAl₁₀O₁₇:Eu²⁺, where M′ is amonovalent cation, and x ranges from about 0.001 to about 0.5.
 8. Thephosphor composition of claim 7, wherein the M′ cation is selected fromthe group consisting of Li, Na, K, Rb, and Cs.
 9. A phosphor compositioncomprising a europium activated BAM phosphor and a hexaaluminate otherthan the BAM phosphor, the composition represented by the formula:(x)M_(0.75)Al₁₁O_(17.25)+(1−x)BaMgAl₁₀O₁₇:Eu²⁺, where M is a divalentcation, and x ranges from about 0.001 to about 0.5.
 10. The phosphorcomposition of claim 9, wherein the hexaaluminate is analkaline-earth-poor hexaaluminate having a β′-alumina structure withabout 75 percent of the group IA alkali metal ions M being replaced bygroup IIA alkaline-earth ions and about 25 percent by oxygen ions.
 11. Aphosphor composition comprising a europium activated BAM phosphor and ahexaaluminate other than the BAM phosphor, the composition representedby the formula:x(yMO·6Al₂O₃)+(1−x)BaMgAl₁₀O₁₇:Eu²⁺, where M is a divalent cation, and xranges from about 0.001 to about 0.5, and y ranges from about 1.28 toabout 1.32.
 12. The phosphor composition of claim 11, wherein thehexaaluminate is an alkaline-earth-rich hexaaluminate having aβ′-alumina structure with about 75 percent of the group IA alkali metalions M being replaced by group IIA alkaline-earth ions and about 25percent by oxygen ions.
 13. A method of synthesizing a phosphorcomposition comprising a europium activated BAM phosphor and ahexaaluminate other than the BAM phosphor, the composition representedby the formula:(x)hexaaluminate+(1−x)BaMgAl₁₀O₁₇:Eu²⁺, wherein x ranges from about0.001 to about 0.999, and wherein the synthesis method is selected fromthe group consisting of liquid processing methods, co-precipitationmethods, and sol-gel methods.
 14. A method of synthesizing a phosphorcomposition comprising a europium activated BAM phosphor and ahexaaluminate other than the BAM phosphor, the composition representedby the formula:(x)hexaaluminate+(1−x)BaMgAl₁₀O₁₇:Eu²⁺, wherein x ranges from about0.001 to about 0.999, and wherein the synthesis method comprises: (a)dissolving the precursor metal salts in an aqueous based solution; (b)co-precipitating an intermediate product; (c) removing at least aportion of the water from step (b); (d) calcining the product of step(c); and (e) sintering the product of step (d).